1. Field of the Invention
The present invention relates to an organic EL element (Organic Electro-Luminescence element or organic light emitting diode) which has a high light extraction efficiency and is capable of preventing the occurrence of a leak current effectively, and a method for manufacturing the same.
2. Description of the Related Art
There has been known a nanoimprint method, in addition to a lithography method, as a method for forming a minute or fine pattern such as a semiconductor integrated circuit. The nanoimprint method is a technique such that a pattern of an order of nanometer can be transferred by sandwiching a resin between a mold and a substrate. A thermal nanoimprint method, a photonanoimprint method, and the like have been studied depending on the employed material. Of the above methods, the photonanoimprint method includes four steps of: i) resin coating (application of a resin layer); ii) pressing by use of the mold; iii) photo-curing; and iv) mold-releasing. The photonanoimprint method is excellent in that processing on a nanoscale can be achieved by the simple process as described above. Especially, since a photo-curable resin curable by being irradiated with light is used as the resin layer, a period of time for a pattern transfer step is short and high throughput is promised. Thus, the photonanoimprint method is expected to come into practical use in many fields including, for example, an optical member such as an organic EL element and LED, MEMS, and a biochip, in addition to a semiconductor device.
In the organic EL element (organic light emitting diode), a hole injected from an anode through a hole injecting layer and electron injected from a cathode through an electron injecting layer are carried to a light emitting layer respectively, then the hole and electron are recombined on an organic molecule in the light emitting layer to excite the organic molecule, and thus light emission occurs. Therefore, in a case that the organic EL element is used as a display device and/or an illumination device, the light from the light emitting layer is required to be efficiently extracted from the surface of the organic EL element. In order to meet this demand, Japanese Patent Application Laid-open No. 2006-236748 discloses that a diffraction-grating substrate having a concave-convex structure is provided on a light extraction surface of the organic EL element.
The light emitted from the light-emitting layer goes outside through an electrode, and thus a thin film made of indium tin oxide (ITO) having a light-transmissive property is generally used in one of a pair of electrodes. The light emitted from the light-emitting layer goes outside through the ITO thin film and a substrate with the ITO thin film formed thereon. The refractive index of the transparent electrode is generally higher than the refractive index of the substrate. For example, the refractive index of the glass substrate is about 1.5 and the refractive index of the ITO thin film is about 2.0. Such a relation between the refractive index of the transparent electrode and the refractive index of the substrate is more likely to cause the total reflection of the light emitted from the light-emitting layer at an interface between the transparent electrode and the substrate. As a result, the light emitted from the light-emitting layer is trapped in an element, which causes a problem such that the efficiency of the extraction of the light from the substrate deteriorates.
Japanese Patent Application Laid-open No. 2011-44296 discloses a method for manufacturing a substrate with electrodes for an organic Electro-Luminescence element, wherein the substrate with electrodes is configured such that a low refractive index layer, a functional layer, and an electrode having a light transmissive property are stacked in this order, and wherein the refractive index n1 of the electrode, the refractive index n2 of the functional layer, and the refractive index n3 of the low refractive index layer satisfy 0≦(n1−n2)≦0.3 and n3<n2≦n1. In this manufacturing method, the difference between the refractive index of the functional layer and the refractive index of the electrode is small, and thus the total reflection at an interface between the functional layer and the electrode is inhibited and the light emitted from a light emitting layer and coming into the electrode is efficiently transmitted to the functional layer. Further, by forming the interface between the functional layer and the low refractive index layer to have a concave-convex shape, the concavities and convexities function as a microlens, which inhibits the total reflection at the interface between the functional layer and the low refractive index layer. Accordingly, the light coming into the functional layer from the electrode is transmitted to the low refractive index layer efficiently. Further, the substrate has a layer structure such that a layer closer to the outside (air) has a smaller refractive index, and thus it is possible to reduce the difference between the refractive index of air and the refractive index of a layer contacting with air (low refractive index layer). Japanese Patent Application Laid-open No. 2011-44296 further discloses that a surface of the functional layer on the electrode side is formed to be flat.
In order to improve the light extraction efficiency and the visibility of an organic EL light-emitting element, Japanese Patent Application Laid-open No. 2011-48937 discloses an organic EL light-emitting element 1 as follows. That is, as depicted in FIG. 11 of Japanese Patent Application Laid-open No. 2011-48937, a first electrode 3, an organic layer 4, and a second electrode 5 are stacked on a substrate 2 in this order. A minute concave-convex structure 6 of which arrangement or array pitches are not more than incoming wavelengths is provided between the substrate 2 and the first electrode 3 on the side of the substrate 2, and a transparent layer 7 is provided on the side of the first electrode 3. The refractive index n1 of the substance constituting the substrate 2 is not less than the refractive index n2 of the substance constituting the minute concave-convex structure (n1≧n2), and thus the difference between the refractive index of the substrate 2 and the refractive index of the first electrode 3 is inclined. Then, the reflection at the interface due to the difference between the refractive indexes is reduced, and thereby making it possible to improve the light extraction efficiency in Japanese Patent Application Laid-open No. 2011-48937. Especially, by filling the concavities and convexities of the minute concave-convex structure 6 to make the transparent layer 7 flat, a thin film of the first electrode 3 formed on the transparent layer 7 does not have uneven film thickness, which removes the possibility of occurrence of a short circuit in Japanese Patent Application Laid-open No. 2011-48937.
In the structure of the organic EL light-emitting element described in each of Japanese Patent Application Laid-open No. 2011-44296 and Japanese Patent Application Laid-open No. 2011-48937, the light extraction efficiency is improved by providing the layer of the minute concave-convex structure on the substrate and providing the layer for adjusting the refractive index which has a planarized or flattened surface between the first electrode and the layer of the minute concave-convex structure. However, the investigation and study of the applicant of the present application showed that the technique described in the above patent literatures did not yet have sufficient light extraction efficiency. Further, in a case that the diffraction-grating substrate having the concave-convex structure is provided on the light extraction surface of the organic EL element, the occurrence of a leak current due to the concave-convex structure is required to be inhibited. Therefore, an organic EL light-emitting element, which has a sufficient light extraction efficiency while inhibiting the occurrence of the leak current, is expected to be developed in order that the organic EL light-emitting element is put into practical use in many fields such as a display and lighting.
In view of the above, an object of the present invention is to provide an organic EL light-emitting element which has a high light extraction efficiency while inhibiting the occurrence of a leak current.